Dispensing syringes that are used to dispense small quantities of liquids are used in many industrial applications, such as dispensing glue, paint, printed circuit board masking material, among other things. Glue may be dispensed onto containers such as cereal box containers or the like. Paint may be dispensed on to pressure cast buttons or lapel pins, or in sign making. Masking material is put on to unfinished printed circuit boards in the pattern of the required circuit before an etching solution is introduced thereto. These are only illustrative examples and several other uses for dispensing syringes exist.
Essentially what happens in the use of a dispensing syringe to dispense a liquid onto an object, is that the dispensing syringe moves along the object with the outlet of the dispensing syringe near the object. The amount of liquid that is required per unit distance along the object is known for any given point along the object. This amount can remain constant for all points along the path of travel of the dispensing syringe or can vary as necessary. If the amount of liquid to be laid down is constant for all points along the path, then it can be considered that the liquid is being laid down at a specific volume per unit distance. Given this known rate of volume of liquid per unit distance and the rate of movement of the outlet of the dispensing syringe along the object, the rate of dispensing of liquid from the outlet of the dispensing syringe can be calculated by multiplying these two values. This calculated volume that must be dispensed from the syringe can be used to calculate the speed of movement of the piston in the syringe as required by dividing the calculated flow rate by the cross-sectional area of the reservoir of the dispensing syringe. Conversely, given this known rate of volume of liquid per unit distance and the rate of dispensing of the liquid from the syringe, the necessary rate of movement of the outlet of the dispensing syringe can be calculated, by dividing the rate of dispensing of the liquid from the syringe by the volume of liquid per unit distance.
Further, the above relationship can also be considered as an equation wherein the rate of dispensing of the dispensing syringe equals a constant times the rate of movement of the outlet of the dispensing syringe, where the constant is the amount of volume of liquid per unit distance which is generally known.
Often, small amounts or even very small amounts of the liquid must be dispensed per unit time and it must be dispensed at an accurate rate which is difficult at small flow rates. There are at least two reasons for this. Firstly, it is desirable to minimize wastage. Secondly, in some applications such as painting buttons or lapel pins, extreme neatness and accuracy is required.
In order to deposit a known amount of liquid over a given distance of travel of the outlet of the dispensing syringe, the amount of movement of the outlet of the dispensing syringe and the amount of liquid dispensed over this given distance must remain in constant proportion to each other. In order to do this, the rate of change in displacement of the outlet along the workpiece and the liquid dispensing rate must remain constant, or alternatively the two must each have the same rate of change. Typically, the rate of dispensing of the liquid from the dispensing syringe is adjusted, possibly by trial and error, with respect to rate of movement of the outlet of the dispensing syringe in order to obtain the desired known amount of deposited liquid per unit distance.
In the case where the amount of material required per unit distance is not constant, it is necessary that the flow rate of the material being dispensed from the outlet of the dispensing syringe be adjustable directly according to the amount required, assuming a constant speed of the dispensing syringe across the object. Lag in the change of dispensing rate or a change in the dispensing rate to an incorrect amount will cause an improper amount of liquid to be deposited on the object. Conversely, the rate of movement of the outlet of the dispensing syringe across may be varied while the rate of dispensing of liquid from the dispensing syringe remains constant. It can be seen, therefore, that it is necessary to consistently and smoothly advance the piston along the cartridge of the dispensing syringe, at a potentially continuously changing speed that changes in proportion to the amount of liquid per unit length to be deposited on an object and also in proportion to the change in speed of the outlet of the dispensing syringe across the object.
Dispensing syringes are often used in hand controlled situations wherein a stylus is hand drawn by an operator along grooves in a template. The outlet of the dispensing syringe follows the same pattern across an object. Inevitably, the speed of the stylus within the groove of the template varies due to the nature of the operator. Again, it is necessary to vary the rate at which the liquid is being dispensed from the outlet of the dispensing syringe, and therefore also the speed of the piston in the cartridge of the dispensing syringe, proportionally with the change in speed of the stylus in the grooves of the template.
Another problem associated with depositing a known amount of liquid per unit distance along an object is that many dispensing syringes are used in conjunction with a two degree of freedom (two independent axes of motion) robotic table, commonly known as an X-Y table. In such a robotic table, either the table or the dispensing syringe is independently driven in each of two mutually perpendicular directions (the X-axis and Y-axis of Cartesian coordinates). In any event, the table and the dispensing syringe are moved with respect to each other. Typically, the two drive means that are used to drive the table or the dispensing syringe with respect to the other in each of the X and Y axes run at constant speeds. The movement in each of the X and Y axes is independent from each other and the resulting speed of the table is the vectorial sum of the two speeds in each of the mutually perpendicular axes. This vectorial sum can be calculated by using the equation SPEED=((SPEED IN "X" DIRECTION).sup.2 +(SPEED IN "Y" DIRECTION).sup.2).sup.1/2. Therefore, in order to move the table and the dispensing syringe with respect to one another in a path that is at an angle to the X and Y axes, the speed of the dispensing syringe is greater than the speed along either of the X and Y axes.
In order to accommodate such a change in speed of the dispensing syringe the rate at which the liquid is dispensed from the dispensing syringe must increase correspondingly, so that the ratio of the speed of the dispensing syringe to the rate of liquid dispensed therefrom remains constant, with this constant being the amount of liquid required per unit distance. It is therefore necessary to change the speed of the piston within the cartridge of the dispensing syringe proportionally to any change in relative speed of the outlet of the dispensing syringe across the object, which relative speed can be calculated as discussed previously.
It is also possible to use a dispensing syringe in conjunction with a three degree of freedom (three independent axes of motion) robotic table, commonly known as an X-Y-Z table. In such a robotic table, the table is independently driven in each of three mutually perpendicular directions (the X axis, the Y axis and the Z axis of Cartesian co-ordinates). As in the above described X-Y table, the speed of the outlet of the dispensing syringe across an object on the X-Y-Z table can be calculated by using the equation SPEED=((SPEED IN "X" DIRECTION).sup.2 +(SPEED IN "Y" DIRECTION).sup.2 +(SPEED IN "Z" DIRECTION).sup.2).sup.1/2. Typically, a separate drive means would be used to drive the table or dispensing syringe with respect to each other in each of the X, Y and Z axes, thereby causing the need for the vectorial sum of the speed in each of these axes to be calculated to obtain the relative speed of the outlet of the dispensing syringe with respect to the object on the X-Y-Z table.